preloadWindow makes dock icon always shows on macOS

Description

On Electron 9.3.0. Setting preloadWindow to true will cause dock icon to be shown even if showDockIcon is set to false (default). It works correctly on Electron 9.2.1

Steps to Reproduce the Problem

const { menubar } = require('menubar');

const mb = menubar({
	preloadWindow: true,
});

mb.on('ready', () => {
	console.log('Menubar app is ready.', process.versions.electron);
});

Expected Behaviour

Dock icon is hidden

Actual Behaviour

Dock icon is shown

Specifications

• Menubar version: 9.0.1
• Platform: macOS
• Electron version: 9.3.0

Other information